1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having a circuit constituted of a thin film transistor (hereafter referred to as a TFT). For example, the present invention relates to an electro-optical device, typically an EL display device or a light-emitting device, and to the structure of electronic equipment in which an electro-optical device is included as a part. Further, the present invention relates to a method of manufacturing the above device. Note that, throughout this specification, the category xe2x80x9csemiconductor devicexe2x80x9d indicates general devices which can function by utilizing semiconductor characteristics, and the above electro-optical devices and electronic equipment are within the semiconductor device category.
2. Description of the Related Art
Research into techniques of increasing crystallinity when implementing laser annealing on an amorphous semiconductor film formed on an insulating substrate such as glass, thereby crystallizing the amorphous semiconductor film, has been widely performed in recent years. Silicon is often used as the amorphous semiconductor film.
Glass substrates are blessed with low cost and good workability compared with the synthetic quartz substrates often used conventionally, and possess an advantage of being easily manufactured into a large surface area substrate. This is the reason that the above research is being carried out. Further, the preferable use of a laser for crystallization is due to the melting point of glass substrates. Lasers are capable of imparting high energy only to the amorphous semiconductor films, without raising the temperature of the substrate very much.
Crystalline semiconductor films are formed from many crystal grains, and therefore they are also referred to as polycrystalline semiconductor films. A crystalline semiconductor film formed by implementing laser annealing has a high mobility, and TFTs are formed using the crystalline semiconductor film, for example TFTs for a pixel portion and a driver circuit portion formed on one glass substrate, are enthusiastically utilized in such devices as a monolithic type liquid crystal electro-optical device.
Further, a high output pulse laser beam such as an excimer laser beam widely used due to the fact that a method for performing laser annealing, in which the laser beam is formed into a square spot shape of several centimeters, or into a linear shape having a length of 10 cm or more by an optical system, on a surface to be irradiated, and then scanned (or the laser beam irradiation apparatus can be moved relative to the surface to be irradiated), has high productivity and superior workability.
In particular, the productivity is high if a linear shape beam is used, because differing from a case of using a spot shape laser beam in which it is necessary to scan forward and backward, and left and right, laser irradiation can be performed over the entire surface to be irradiated by scanning only in a direction perpendicular to the longitudinal direction of the linear shape beam. Scanning in a direction perpendicular to the longitudinal direction is performed because that is the scanning direction having the maximum efficiency. The use of a linear shape beam, formed by an appropriate optical system from a pulse emission excimer laser beam, in the laser annealing method at present due to superior productivity is becoming the main production technique for liquid crystal display devices which use TFTs. This technique makes possible a monolithic type liquid crystal display device in which TFTs forming a pixel portion (pixel TFTs), and driver circuit TFTs formed in the periphery of the pixel portion, are all formed on one glass substrate.
However, a crystalline semiconductor film manufactured by the laser annealing method is formed by a plurality of crystal grains, and the position and size of the crystal grains is random. The TFTs formed on the glass substrate are separated by element, and therefore formed by separating the crystalline semiconductor film into island shape patterns. The size and the position of the grains cannot be set in this case. Compared to within a crystal grain, there are an almost limitless number of re-crystallization centers and capture centers in the boundaries of the crystal grains (grain boundaries) which are the cause of amorphous structure and crystal defects. If a carrier is trapped in a capture center, then the potential of the grain boundary increases and this becomes a barrier with respect to the carrier, and it is known that the electric current transporting characteristics therefore drop. The crystallinity of the semiconductor film of a channel forming region has a great influence on the TFT properties, but it is nearly impossible to form the channel forming region by a single crystal semiconductor film, eliminating the effect of the grain boundaries.
In order to solve this type of problem, position is controlled for laser annealing, and various tests have been performed for forming large size crystal grains. A solidification process of the semiconductor film after the laser beam is irradiated to the semiconductor film is explained here first.
A certain amount of time is necessary until crystal nuclei develop within a semiconductor film which has been completely melted by laser beam irradiation, a large number of crystal nuclei are generated uniformly (or non-uniformly) in the completely melted region, and the solidification process of the completely melted semiconductor film is completed by crystal growth. The position and the size of the crystal grains obtained in this case becomes random.
Further, for cases in which the semiconductor film is not completely melted by the laser beam irradiation and solid semiconductor regions remain partially, crystal growth begins from the solid semiconductor regions immediately after laser beam irradiation. As stated above, a certain amount of time is necessary until crystal nuclei develop in the completely melted region. Thus, during the period until crystal nuclei develop in the completely melted region, the solid-liquid interface (indicating the interface between the solid semiconductor region and the completely melted region), which is the crystal growth leading edge, moves in a direction parallel to the film surface of the semiconductor film (hereafter referred to as a lateral direction), and crystal grains grow to a length several tens of times longer than the film thickness. A very large number of crystal nuclei develop uniformly (or non-uniformly) in the completely melted region with this type of growth, which is completed by crystal growth. This type of phenomenon is hereafter referred to as xe2x80x9csuper lateral growth.xe2x80x9d
A laser beam energy region for also achieving super lateral growth in amorphous semiconductor films and in polycrystalline semiconductor films exists. However, this energy region is extremely narrow, and positions at which large size crystal grains are obtained cannot be controlled. In addition, microcrystalline regions, in which a very large number of crystal nuclei develop, and amorphous regions exist in regions outside the large size crystal grains.
As explained above, the position of grain growth and the growth direction can be controlled provided that the lateral direction temperature gradient is controlled by a laser beam energy region in which the semiconductor film is completely melted (making heat flow arise in a lateral direction). Several tests have been performed in order to realize this method.
For example, Ishihara, R., and Burtsev, A., (AM-LCD ""98, pp. 153-156, 1998) reported on a laser annealing method in which they formed a high melting point metallic film between a substrate and a base silicon oxide film, and formed an amorphous silicon film above the high melting point metallic film, and then irradiated an excimer laser beam from both the top surface side of the substrate (defined in this specification as the face upon which the film is formed) and from the bottom surface side of the substrate (defined in this specification as the face on the opposite side as the face upon which the film is formed). The laser beam which is irradiated from the top surface of the substrate is absorbed by the silicon film and its energy is converted into heat. On the other hand, the laser beam which is irradiated from the bottom surface is absorbed by the high melting point metallic film and its energy is converted to heat; the high melting point metallic film is heated to a high temperature. The silicon oxide film between the heated high melting point metallic film and the silicon film works as a heat accumulation layer, and therefore the cooling speed of the melted silicon film can be slowed. It is reported that crystal grains having a maximum diameter of 6.4 xcexcm can be in arbitrary locations by forming the high melting point metallic film in the arbitrary locations.
James S. Im, et al., of Columbia University showed a sequential lateral solidification method (hereafter referred to as SLS method) in which super lateral growth can be achieved in arbitrary locations. The SLS method is one in which crystallization is performed by moving a slit shaped mask over a distance on the order of which super lateral growth takes place (approximately 0.75 xcexcm) every shot.
In addition, Masakiyo Matsumura, et al., of the Tokyo Institute of Technology reported, at the 47th Applied Physics Society Symposium, a method of forming large size crystal grains which are controlled by position. With this method, an insulating layer having a quadrilateral top surface shape and having at least one vertex with an angle of 60xc2x0 is embedded within an amorphous silicon film, as shown in FIG. 5C. In addition, an insulating film is formed on the amorphous silicon film. A phase shift mask (see FIG. 5A) is used during laser beam irradiation, and the laser beam is made to possess an energy gradient (see FIG. 5B). Provided that a temperature gradient is formed within the amorphous silicon film, crystal nuclei develop within the amorphous silicon film under the insulating layer, and therefore large size crystal grains are formed at a controlled position.
It is structurally possible to manufacture a top gate TFT with a semiconductor film, formed in accordance with the method of Ishihara, et al., as an active layer. However, a parasitic capacitance develops in accordance with the silicon oxide film formed between the semiconductor film and the high melting point metallic film, and energy consumption is therefore increased, and it becomes difficult to achieve high speed TFT operation. On the other hand, it is thought that this method can be effectively applied to a bottom gate or inverse stagger TFT by using the high melting point metallic film as a gate electrode. However, with a structure in which a silicon oxide film is formed on a substrate, then a high melting point metallic film is formed on the silicon oxide film, and an amorphous silicon film is formed on the high melting point metallic film. Even if the thickness of the amorphous silicon film is not considered, the film thickness of the high melting point metallic film and that of the silicon oxide film is such that the film thickness suitable for the crystallization process is not necessarily in agreement with that suitable for good TFT element properties. Optimal design in the crystallization process and optimal design of the element structure therefore cannot both be satisfied at the same time.
Further, if a non light transmitting high melting point metallic film is formed over the entire surface of the glass substrate, then it becomes impossible to manufacture a transmission type liquid crystal display device. The internal stress of a chrome (Cr) film or titanium (Ti) film used as the high melting point metallic material is high, and therefore there is a high likelihood that problems will develop with the adherence of these films to a glass substrate. In addition, the internal stress influence also affects the semiconductor film formed above this layer, and there is a high likelihood of a force acting to distort the formed crystalline semiconductor film.
On the other hand, in order to control the threshold voltage (hereafter denoted by Vth), a very important parameter in TFTs, to be within a predetermined range, it is necessary to control the electric charge of the channel forming region, and in addition, to consider how to reduce the charge defect density of a base film formed by an insulating film in contact with the active layer or a gate insulating film, and how to balance the internal stress. A material containing silicon as a structural element, such as a silicon oxide film and a silicon oxynitride film, is appropriate for these demands. There is a worry that formation of the high temperature metallic film between the substrate and the base film will upset that balance.
Further, precise control on a micron order is required for a technique of determining the relative position between the mask and the substrate with the SLS method, and this will become a complex apparatus compared to a conventional laser irradiation apparatus. In addition, there is a problem with throughput in using this method to manufacture TFTs which will be applied to a liquid crystal display having a large surface area region.
With the method announced by Matsumura, et al., it is necessary to use a phase shift mask in order to make an energy gradient in the laser beam. It is therefore necessary to have precise control on the micron order for a technique of determining a relative position between the phase shift mask and the embedded insulating layer, and this becomes a complex apparatus compared to a conventional laser irradiation apparatus. Further, the top surface shape of the embedded layer is a quadrilateral, and at least one vertex of the quadrilateral is opened to 60xc2x0, and therefore a plurality of crystal nuclei develop within the semiconductor film existing below in the vicinity of the vertex when the semiconductor film which has been irradiated by the laser beam is cooled from a melted state. A problem consequently develops in which growing crystal grains collide with each other, reducing the probability of forming large size crystal grains.
The present invention is one for solving these types of problems, and an object of the present invention is to realize a TFT capable of high speed operation by manufacturing a crystalline semiconductor film in which the position and size of the crystal grains are controlled, and further by using the crystalline semiconductor film as a channel forming region of the TFT. In addition, an object of the present invention is to provide a technique in which this type of TFT can be applied to various types of semiconductor devices such as transmission type liquid crystal display devices and display devices using an electro-luminescence material. The EL (electro-luminescence) materials referred to in this specification include triplet-based light emission devices and/or singlet-based light emission devices, for example.
The reflectivity during irradiation of a laser beam from an insulating film formed on a semiconductor film is explained. Examples are explained here in which an amorphous silicon film and a silicon oxide film are used as the semiconductor film and the insulating film, respectively, and the wavelength of the laser beam is taken as 308 nm and 532 nm, but there are no particular limitation placed upon the semiconductor film, the insulating film, or the wavelength of the laser beam in the present invention.
FIG. 1A shows the changes in reflectivity when an XeCl excimer laser (wavelength 308 nm) is irradiated on a silicon oxide film, with the film thickness of the silicon oxide film as a parameter. It can be seen that the reflectivity of the XeCl excimer laser with respect to the silicon oxide film changes periodically in a range from 26% to 56% in accordance with the film thickness of the silicon oxide film.
Further, when one wants to change the effective irradiation strength of the laser beam with respect to the semiconductor film by forming an insulating film on portions of the semiconductor film, it becomes necessary to also consider the reflectivity of the semiconductor film.
FIG. 1B shows the change in reflectivity when the XeCl excimer laser (wavelength 308 nm) is irradiated on an amorphous silicon film, with the film thickness of the amorphous silicon film as a parameter. With the film thickness of the amorphous silicon film on the order of 5 nm or less, the reflectivity is lower than the minimum reflectivity (26%) obtained when irradiating the XeCl excimer laser on the silicon oxide film and changing the film thickness of the silicon oxide film. Further, the reflectivity when the film thickness of the amorphous silicon film is from 5 to 12 nm is in the same range (26% to 56%) as the reflectivity obtained when irradiating the XeCl excimer laser on the silicon oxide film and changing the film thickness of the silicon oxide film. It is therefore necessary to choose a film thickness of the silicon oxide film in correspondence with the film thickness of the amorphous silicon film for cases in which the effective irradiation strength of the XeCl excimer laser with respect to the amorphous silicon film is changed. When the film thickness of the amorphous silicon film exceeds 12 nm, the reflectivity is on the same order as the maximum reflectivity (56%) obtained when irradiating the XeCl excimer laser on the silicon oxide film while changing the film thickness of the silicon oxide film. The reflectivity also becomes higher than 56%.
Changes in reflectivity when irradiating a laser beam having a 532 nm wavelength are shown next. FIG. 2A shows the changes in reflectivity when the second harmonic (wavelength 532 nm) of a YAG laser is irradiated on a silicon oxide film, with the film thickness of the silicon oxide film taken as a parameter. FIG. 2B shows the changes in reflectivity when the second harmonic of the YAG laser is irradiated on an amorphous silicon film, with the film thickness of the amorphous silicon film taken as a parameter. As shown in Table 1, the 532 nm wavelength laser beam has a lower attenuation constant than the 308 nm wavelength laser beam with respect to the amorphous silicon film, and therefore the reflectivity when irradiated on the silicon oxide film differs in accordance with the film thickness of the amorphous silicon film existing below the silicon oxide film. The film thickness of the amorphous silicon film is set to 58 nm in FIG. 2A.
The reflectivity changes periodically in FIG. 2A, similar to FIG. 1A. The reflectivity shows a tendency to converge while changing periodically as the film thickness of the amorphous silicon film becomes thicker, as shown in FIG. 2B. Further, the reflectivity of the silicon oxide film with respect to the 532 nm wavelength can be seen from FIG. 2A and FIG. 2B to be on the same order as, or less than, the reflectivity of the amorphous silicon film.
In other words, the insulating film has a reflection preventing effect and a heat insulating effect when irradiating the laser beam, provided that the film thickness of an insulating film is given a film thickness having a low laser beam reflectivity when forming the insulating film on a semiconductor film. The semiconductor film can therefore be maintained for a long period in a melted state. Further, if an insulating layer is formed on portions of the semiconductor film, then it is necessary to set the film thickness while considering the reflectivity of the semiconductor film and the reflectivity of the insulating layer when changing the effective irradiation strength of the laser beam with respect to the semiconductor film. In addition, the reflectivity also changes in accordance with the wavelength of the laser beam, and therefore it is necessary to set the film thicknesses corresponding to the laser beam wavelength. It should be noted that the heat insulating effect in this specification means that a melted state of a semiconductor film on which an insulating film is formed is maintained longer time than a semiconductor film on which an insulating film is not formed after laser beam is irradiated to the semiconductor film.
A structure is shown in FIG. 5C in which an insulating layer (embedded insulating layer) having a top surface shape which is quadrilateral, and having at least one vertex with an angle of 60xc2x0, exists within a semiconductor film. The 60xc2x0 angle of the vertex is wide, and therefore a plurality of crystal nuclei develop within the semiconductor film existing below and in the vicinity of the vertex when a laser beam is irradiated. Growing crystal grains consequently collide with each other, and the probability of forming large size crystal grains becomes low. In other words, in order to form large size crystal grains, the density of crystal nuclei developing under a vertex will become lower provided that the angle of at least one vertex is less than 60xc2x0 when seen from the top surface of the embedded insulating layer. The mutual collision of growing crystal grains can be reduced.
The top surface shape of an insulating layer embedded within a semiconductor film is thus set in the present invention to be a polygonal shape having at least one vertex with an angle of less than 60xc2x0. In addition, an insulating film is formed on the semiconductor film, a region overlapping with the embedded insulating layer is etched to form an insulating layer, and the insulating layer is given a reflection prevention effect and a heat insulating effect during irradiation of a laser beam. A crystalline semiconductor film having large size crystal grains in controlled positions is formed. Note that, the irradiation of the laser beam is performed from the top surface side of the substrate, or from both the top surface side and the bottom surface side of the substrate.